Joseph M. O’Connor

Cobalt 1,3-Diisopropyl-1H-imidazol-2-ylidene Complexes: Synthesis, Solid-State Structures, and Quantum Chemistry Calculations

The reaction of (η5-C5H5)Co(PPh3)2 (1) with 1,3-bis(isopropyl)imidazol-2-ylidene (ImiPr2, 17) leads to the formation of (η5-C5H5)Co(PPh3)(ImiPr2) (5) in 69% yield. N-Heterocyclic carbene 17 also undergoes reaction with (η5-C5H5)Co(CO)2 (9) to give (η5-C5H5)Co(ImiPr2)(CO) (6) in 30% yield. The barrier to rotation about the Co−Ccarbene bond in 6 has been determined by variable-temperature 1H NMR spectroscopy (13.6 kcal/mol) and by computation (13.3 kcal/mol). Complex 5 undergoes reaction with PhSSPh to give the paramagnetic thiolato complex (η5-C5H5)Co(ImiPr2)(SPh) (7), which is oxidized to the metallosulfone complex (η5-C5H5)Co(ImiPr2)(SO2Ph) (8). The solid-state structures of 5−8 were determined by X-ray crystallography. The structural and dynamic properties of 6, (η5-C5H5)Co(ImMe2)(CO) (ImMe2 = 1,3-dimethylimidazol-2-ylidene), and (η5-C5H5)Co(ImAr2)(CO) (ImAr2 = 1,3-dimesityl-2-ylidene) were examined by quantum chemistry calculations.

Acceleration of conjugated dienyne cycloaromatization.

The thermal cycloaromatization of conjugated 1,3-dien5-ynes has long been an area of intense interest among physical organic and computational chemists. Hopf and Musso initially reported that heating (Z)-1,3-hexadien-5-yne (1) at temperatures in excess of 274 C resulted in conversion to benzene (2, Scheme 1). At temperatures greater than 550 C, several mechanisms may be active; however, computational and experimental studies provide strong support for the mechanism shown in Scheme 1 as the most dominate one at lower temperatures. From 1, electrocyclization leads to cyclic allene intermediate 3 that then proceeds through an initial [1,2]-H shift to afford intermediate 4 which can be represented as either carbene 4-A or diradical 4-B. A final [1,2]-H shift leads to the aromatized product 2. While the electrocyclization pathway shown in Scheme 1 represents the most well studied thermal dienyne cycloaromatization, a lesser-known variant initiates via a [1,7]-H shift from a cis-allylic substituted dienyne 5 to give allene intermediate 6 (Scheme 2). 6π-Electrocyclization of 6 gives 7, which in turn isomerizes to 8 via a [1,3]-H shift. Although thermal [1,7]-H shifts are well-known for 1,3,5-hexatrienes, examples with dienynes are rare and generally occur at high temperature (>200 C). Synthetically, dienyne cycloaromatization provides a reliable way to construct highly substituted aromatic systems from readily available starting materials as depicted in Scheme 3. A major drawback of the thermal mode of cyclization is the exceedingly high temperatures required to effect cyclization, thus limiting the substrate scope. The motive of this review is to highlight the discovery, mechanism, and synthetic utility of methodologies that use either catalytic or stoichiometric activators to promote dienyne cycloaromatizations at temperatures below 200 C. The review is organized by mechanism of activation and covers cyclizations resulting in carbon-based aromatic systems (e.g., benzenoid, naphthalenoid). Heterocyclic aromatic systems are not reviewed unless a mechanistic discussion is warranted. In order to decrease redundancy throughout the discussion, dienyne substrates have been classified according to the presence and location of aromatic alkene subunits as shown in Figure 1.